The “air knife” method is a well-known process used to wipe the excess of liquid entrained by a running strip going out of a bath. A typical air knife uses chamber pressure as high as 700 mbar leading to an exit gas velocity close to the sound level. The air discharge opening is usually in the range of 0.5 to 2 mm.
The air wiping process generates some waves in the coating due to the high turbulence occurring when the gas jet enters in the atmosphere. This high turbulence cannot be reduced due to the high level of shear forces. However, those waves tend to be reduced in amplitude with time due to the levelling process that occurs in the liquid state, driven by the liquid surface tension.
A countermeasure to limit the amplitude of those waves on the aspect of the finished product consisting for example in a zinc coated steel sheet, resides in the replacement of air as wiping medium by nitrogen (N2). This method indeed induces a significant reduction of the oxidation of the liquid coating and helps keeping a high surface tension. Since the surface tension of the liquid metal is kept high, the final surface after solidification is a smoother surface when N2 is used. This leads to a much better surface appearance after painting. A typical case is that of the galvanized steel sheets used for the exposed panels in automobiles.
In case of low line speed, air can generate defects such as those shown on FIG. 1 that are suspected to be due to the metal oxidation. Again, N2 wiping helps to significantly reduce these defects.
Finally, air wiping can induce what is called a “cloudy aspect” as shown in FIG. 2 and which is due to differential oxidation of the surface. Here again, N2 wiping is used to drastically improve such a poor surface quality.
A related problem is that the use of N2 is expensive since a flow as high as 800 Nm3/h and per meter of nozzle length may be used. The cost becomes especially high in case of wiping narrow sheets because the gas exits the nozzle opening along the whole length of the nozzle whereas wiping is of course only required in front of the strip. All the N2 flow that is outside the strip is indeed lost.
The solution to reduce those losses and so reduce the operating costs consists in a flexible closing of the air discharge opening in the region where the gas has no wiping effect. To that purpose different methods have been proposed like:                reduction of the nozzle opening through an action on the lips of the nozzle 1 by a mechanical means. FIG. 3, for example, shows such a typical opening of this type;        using a foil inside the nozzle as shown on FIG. 4, the foil being referenced 2 on the figure. The foil 2 is moved by motors located at the edges of the nozzle (not shown), which means that the foil is pushed when the opening 4 must be closed or reduced (see EP 0 249 234 A1, Blow-off device for the continuous two-sided coating of strip metal, Duma Konstructionsbuero)        
The previous methods present various drawbacks due to the operating window used in production as well as the requirements on the final coating as described here below:                the mechanical closing of the opening has side effects on the control of the opening in front of the strip, which impacts the control of the final coating thickness. In addition, due to mechanical constraints, the deformation of the lips should be limited to avoid their plastic deformation;        the foil suffers from the force it has to resist. To take an example, when a chamber pressure of about 600 mbar is used, the force on the surface 2 (FIG. 4) measuring only 5 mm high for example is 120 N on a 400 mm long mask. This means a friction force when the latter is moved of at least 12 N. As the foil is usually thin, it cannot be pushed along the nozzle without buckling;        at the location where the gas flow is stopped due to the masking device, the nozzle temperature increases because it is not cooled anymore by the gas, while being still heated by the radiation of the liquid metal. This leads to a thermal expansion and deformation of the opening all along the nozzle, due to the temperature gradients. This deformation can either be elastic, which would not be a too critical situation, or plastic, depending on the nozzle design, which will impact the coating weight uniformity in this case.        
Document U.S. Pat. No. 4,524,716 A discloses an adjustable gas knife, comprising: elongated nozzle means having an elongated nozzle opening for projecting a sheet of gas; elongated flexible gas flow modification means located within said nozzle for modifying the rate of flow of said gas; and differential adjusting means for selectively adjusting the position of said gas flow modification means relative to said nozzle opening at a plurality of positions along the length of said gas flow modification means to thereby selectively modify the rate of flow of said gas through said nozzle opening.